1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine having an intermediate transfer belt or a printer having an intermediate transfer belt, in which an image is formed by electrostatically transferring an image formed on an image bearing belt onto a transfer material.
2. Related Background Art
In conventional color image forming apparatuses, various systems such as electrophotographic systems, heat-transfer systems, ink jet systems or the like have been utilized. Among them, image forming apparatuses having the electrophotographic system are superior to other image forming apparatuses from a view point of high speed operation, high image quality and silentness and have recently been used widely.
In such electrophotographic image forming apparatuses, there have been used various methods such as a multi-developing method in which, after color images are superimposed on a surface of a photosensitive member, the images are collectively transferred for image formation, a multi-transfer method in which a development/transfer cycle is repeated, or an intermediate transfer method in which, after various color developed images were once transferred onto an intermediate transfer member successively, the images are collectively transferred onto a transfer material. Among them, the intermediate transfer method has been noticed in the points that there is no color mixing between developing devices and that it can be applied to various media.
The intermediate transfer member may be of roller type or of belt type. An intermediate transfer belt is superior to an intermediate transfer roller in the points that it has greater flexibility than the intermediate transfer roller and that separation ability between the transfer material and the belt (after second transferring) is excellent due to the fact that a curvature of the belt can be increased at a second transfer position where the developed images are collectively transferred onto the transfer material.
In general, the intermediate transfer belt is formed from a resin film made of PVdF, nylon, PET or polycarbonate and having a thickness of 100 to 200 .mu.m and volume resistivity of about 10.sup.11 to 10.sup.16 .OMEGA.m. By using such a thin resin film, since great electrostatic capacity of the order of several hundreds to several thousands of pF can be obtained at a transfer nip, stable transfer current can be achieved.
However, when the intermediate transfer belt having the thickness of 200 .mu.m or less is repeatedly flexed by support rollers during rotation, wrinkles are formed on the surface of the belt, thereby causing the image uneven. Further, since the belt may be torn through the wrinkles, a service life of the belt is decreased. In addition, since the resin film cannot be extended, if instantaneous great tension is applied to the belt, the belt cannot absorb such a great force, with the result that the belt will be torn. The image forming apparatus is frequently stopped instantaneously due to sheet jam treatment, or inadvertent door open caused by an operator's erroneous operation. In such a case, the intermediate transfer belt may be torn.
Further, if the thickness of the resin film is increased to improve the service life of the belt, the belt cannot follow the driving roller and/or a driven roller to make the rotation of the belt unstable, with the result that misalignment of registration occurs, thereby worsening the image quality of the color image. In addition, since a friction force is small, slip is easily generated, thereby making the drive unstable.